The objective of this work was to investigate the existence of polymorphs and pseudopolymorphs of aceclofenac which is a peripherally acting, non-steroidal anti-inflammatory analgesic with anti-inflammatory, antiarthritic and antipyretic activities. Three crystal forms of aceclofenac have been isolated by recrystallization and characterized by differential scanning calorimetry (DSC), powder X-ray diffractometry (PXRD) and thermogravimetric analysis. The DSC and PXRD patterns of the three crystal forms of aceclofenac were different respectively. After storage of 1 month at 0 % RH (silica gel, $20^{circ}C$), 52 % RH (saturated solution of $Na_2Cr_2O7{cdot}2H_2O/20^{circ}C$) and 95 % RH (saturated solution of $Na_2HPO_4/20^{circ}C$), Form 1 and Form 3 (from methylethyl ketone) were not transformed, but Form 2 (from DMSO) was transformed to Form 1 at 95 % RH.